WIP: Cygwin build support

[corngood]

Here are some changes I'm working on to make it easier to do cygwin builds without patching the makefile.

With these changes I still need to use:

EXESUFFIX=.exe
LINKER_SOSUFFIX=dll
LIB_SO=cyglowdown.dll
IMPLIB=liblowdown.dll.a
LDFLAGS=-Wl,--out-implib,liblowdown.dll.a

I'm looking for feedback on how to implement these in a merge-able way. It would be nice if configure handled it in some way, but I don't see any similar OS detection in there.

[kristapsdz]

This should really be put into oconfigure, which does the OS/feature detection for this and other projects, and could set those values automatically. I'd love to see cygwin support in there, but I've never had an environment to test it out on.